bhyvectl: improve options error handling

Currently, it is possible to execute bhyvectl(8) with mutually exclusive
options, such as "--destroy" and "--suspend", and it will print out
obscure errors, e.g.:

 bhyvectl --suspend=/var/run/vms/my_vm --destroy --vm my_vm
 connect() failed: Connection refused

Address that by failing early if mutually exclusive options were
specified.

Additionally, move the BHYVE_SNAPSHOT block before the errors are
printed, so its errors are also displayed.

Approved by:		markj
Sponsored by:		The FreeBSD Foundation
MFC after:		2 weeks
Differential Revision:	https://reviews.freebsd.org/D54092

usr.sbin/bhyvectl/bhyvectl.c

@@ -326,7 +326,7 @@ int
 main(int argc, char *argv[])
 {
 	char *vmname;
-	int error, ch, vcpuid;
+	int action_opts, error, ch, vcpuid;
 	struct vm_run vmrun;
 	struct vmctx *ctx;
 	struct vcpu *vcpu;
@@ -338,6 +338,7 @@ main(int argc, char *argv[])
 
 	opts = setup_options();
 
+	action_opts = 0;
 	vcpuid = 0;
 	vmname = NULL;
 	progname = basename(argv[0]);
@@ -388,6 +389,14 @@ main(int argc, char *argv[])
 	if (vmname == NULL)
 		usage(opts);
 
+	action_opts = create + destroy + force_reset + force_poweroff;
+	if (checkpoint_file)
+		action_opts++;
+
+	if (action_opts > 1) {
+		fprintf(stderr, "mutually exclusive actions specified\n");
+		exit(1);
+	}
 
 	ctx = vm_openf(vmname, create ? VMMAPI_OPEN_CREATE : 0);
 	if (ctx == NULL) {
@@ -508,17 +517,17 @@ main(int argc, char *argv[])
 	if (!error && force_poweroff)
 		error = vm_suspend(ctx, VM_SUSPEND_POWEROFF);
 
+#ifdef BHYVE_SNAPSHOT
+	if (!error && checkpoint_file)
+		error = snapshot_request(vmname, checkpoint_file, vm_suspend_opt);
+#endif
+
 	if (error)
 		printf("errno = %d\n", errno);
 
 	if (!error && destroy)
 		vm_destroy(ctx);
 
-#ifdef BHYVE_SNAPSHOT
-	if (!error && checkpoint_file)
-		error = snapshot_request(vmname, checkpoint_file, vm_suspend_opt);
-#endif
-
 	free(opts);
 	exit(error);
 }